Optical amplification units are used e.g. in fiber optical transmission systems. Such systems normally comprise an optical fiber line with a plurality of optical fiber spans interconnected by a plurality of these optical amplification units.
In such optical transmission systems, Wavelength Division Multiplexing (WDM), in particular Dense WDM (DWDM), permits simultaneous transmission of multiple channels over a common optical fiber line. Erbium Doped Fiber Amplifiers (EDFA) provide a cost-effective means for optical amplification in such systems, as EDFAs amplify all the wavelengths simultaneously, enabling the composite optical signals to travel large distances without regeneration.
However, transmission power is influenced by fiber attenuation which is partly due to Rayleigh scattering. The fiber attenuation is responsible for a transfer of power from lower wavelength channels to higher wavelength channels. Consequently, lower wavelength channels encounter a higher span loss in an optical fiber span than higher wavelength channels, thus causing a span loss tilt in the per channel loss in an optical fiber span, see e.g. FIG. 6 for the span loss dependence of wavelength channels in a wavelength range from 1530 nm to 1565 nm.
The number of wavelength channels may vary due to a fiber break for example, when only a few channels remain. The number of channels may also vary in fiber optical systems, in particular ultra long haul (ULH) optical systems with multiple reconfigurable optical add/drop multiplexing (ROADM). In ROADM systems the number of multiplexed frequency channels may vary, e.g. between 1 and 80.
If the optical amplification unit works in constant gain mode, the operating point of the optical amplification unit is conventionally determined to compensate for the average span loss in full load configuration, when all or most of the channels are present. The span loss tilt is then conventionally taken into account by tilting the gain of the optical amplifier using a filter, e.g. a variable optical attenuator (VOA) exhibiting a constant loss versus wavelength, comprised in the optical amplifier. A VOA is usually used within the interstage of an EDFA to adjust the EDFA tilt. By changing the attenuation of the VOA, the overall gain of the EDFA changes (if the input power and the output power are kept constant, the output power depends mainly of the pump power set within the EDFA), and generates a tilt (the inversion parameter of the Erbium doped fiber changes and it modifies the gain shape).
The operating point for an optical amplification unit which compensates for the average span loss in the full load configuration is 19.9 dB in the example of FIG. 6. This operating point conventionally remains constant, even when the load configuration is changed and the number of channels carried by the fiber optical system is changed. If, for example, only a few channels are present around e.g. 1530 nm, the span loss encountered by these channels due to the span loss tilt will be 20.2 dB (cf. FIG. 6). As the operating point of the optical amplification unit is 19.9 dB in this example, the optical amplification unit will still be controlled to have a gain of 19.9 dB. Therefore, the gain error will be 0.3 dB per fiber span in this example case. After 20 optical amplification units and following fiber spans, the accumulated error will be 6 dB, which is not acceptable. Tilting the gain of the optical amplifier, e. g. using a VOA, has not the desired compensation effect, as the remaining channels are located at the same wavelength.
In U.S. Pat. No. 6,437,906 B1, an optical amplifier for a wavelength division multiplexing (WDM) system is described that comprises a filter for inverting the tilt of the WDM signals. The filter is. All the EDFAs installed contain a Gain Flattening Filter (GFF) as a part of the control loop of the optical amplifier to minimize the gain excursion versus the wavelength generated by the natural gain shape of the Erbium doped Fiber.